Proposed is a competing renewal application to continue evaluation of the effectiveness of the Baltimore Needle Exchange Program (NEP). The evaluation is in terms of behavioral risk reduction, HIV seroincidence, and comparisons of characteristics and utilization patterns between those who use a van-based versus pharmacy-based NEP. The Baltimore City Health Department has operated NEP vans in East and West Baltimore since 1994 and will open two pharmacy-based NEPs in West Baltimore in 1997. All NEP sites will offer unlimited one-for-one exchange. Data on drug use behaviors and HIV testing are obtained semi-annually on 2,000 injection drug users, throughout the city, who are participants in ongoing longitudinal studies. Analyses use person-time techniques to compare relative incidence by NEP participation, and also by access versus no access to NEP (i.e. Baltimore city versus county). To address the second generation question of which types of programs attract what types of clients (and which is most effective), we will collect demographic, drug use and NEP utilization information on all participants at all sites. We will compare characteristics of pharmacies versus van in West Baltimore; vans in East and West Baltimore before and after opening pharmacy NEP; and stable van users versus those who switch over to pharmacies. This study will provide new information on the impact of NEP in a community of IDUs, as well as define how different types of programs attract different clientele. This information will provide all NEP services with information enabling them to tailor services and optimize HIV risk reduction.